1. Field of the Invention
The present invention relates generally to cryogenic refrigerators, and more particularly to a cryogenic refrigerator that includes a connecting mechanism that connects a drive unit and a displacer.
2. Description of the Related Art
In general, Gifford-McMahon (GM) refrigerators (hereinafter referred to as “GM refrigerators”) are known as refrigerators that produce cryogenic temperatures. The GM refrigerator produces a cooling effect based on the Gifford-McMahon cycle in which the movement of a displacer containing a regenerator material and the adiabatic expansion of a refrigerant gas due to valve operations are linked with each other.
First, the GM refrigerator feeds a cylinder with a refrigerant gas whose pressure has been increased by a compressor. At this point, the displacer is at its bottom dead center. The displacer is caused to rise by a pressure difference in the refrigerant gas or by the force of a motor. When the displacer reaches its top dead center, the valve is switched to cause the refrigerant gas that has accumulated under the displacer to adiabatically expand, there by causing the refrigerant gas to be cooled and exchange heat with the regenerator material contained in the displacer. At this point, the displacer starts to lower, and when the displacer returns to the bottom dead center, the valve is switched to allow the refrigerant gas whose pressure has been increased by the compressor to reenter the cylinder, so that the refrigerant gas exchanges heat with the regenerator material inside the displacer to be cooled. By repeating this operation, a thermal load flange part on a lower end part of the cylinder is cooled. In general, the rotational motion of a motor is converted into a linear motion using a crank mechanism or a Scotch yoke mechanism, thereby causing the displacer to reciprocate. (See, for example, Japanese Laid-Open Patent Application No. 2007-205582.)
Conventionally, a connecting mechanism as illustrated in FIG. 1 and FIG. 2 is used to connect the reciprocating output shaft of the Scotch yoke mechanism to the displacer. The connecting mechanism includes a collar 102, a parallel pin 104, an upper cup 105, and a spring pin 107.
An output shaft 101 is a rod-shaped member, and is connected to a Scotch yoke mechanism (not graphically illustrated) to reciprocate in vertical directions as shown in FIG. 1. The annular collar 102 fits to a lower end part of the output shaft 101, and is fixed to the lower end part by the parallel pin 104 that passes through the collar 102 and the output shaft 101.
A shaft hole 103a is formed at the upper end of a displacer 103. The output shaft 101 and the collar 102 are inserted in the shaft hole 103a. Further, the upper cup 105 is fixed on the upper end face of the displacer 103 by a fixing bolt 106.
An opening 105a is formed in the center of the upper cup 105. The output shaft 101 extends upward through the opening 105a. Further, the diameter of the collar 102 is greater than the diameter of the opening 105a. 
According to the above-described configuration, when the output shaft 101 moves upward in FIG. 1, the upper surface of the collar 102 is pulled in engagement with the upper cup 105, so that the displacer 103 moves upward inside a cylinder 100. Meanwhile, when the output shaft 101 moves downward in FIG. 1, the displacer 103 is pressed downward by the collar 102, so that the displacer 103 moves downward inside the cylinder 100. Thereby, the displacer 103 reciprocates inside the cylinder 100.
Further, the GM refrigerator produces cold temperatures by expanding a refrigerant gas inside the cylinder 100. Therefore, if the refrigerant gas flows between the inner wall surface of the cylinder 100 and the outer wall surface of the displacer 103 (that is, if a so-called blow-through of the refrigerant gas occurs), this causes a decrease in the cooling efficiency. Therefore, a sealing member 108 that comes into sliding contact with the cylinder 100 is provided on the side surface of the displacer 103 to prevent occurrence of a blow-through of the refrigerant gas.
If the displacer 103 rotates on a vertical axis during its reciprocation inside the cylinder 100, this changes the position of contact of the sealing member 108 provided on the side surface of the displacer 103 with the interior circumferential surface of the cylinder 100. This causes occurrence of a blow-through of the refrigerant gas, thus destabilizing cooling by the GM refrigerator. In order to prevent this, the above-described connecting mechanism is provided with a mechanism to prevent the rotation of the displacer 103 (a rotation prevention mechanism).
As illustrated in FIG. 2 as well as FIG. 1, according to the conventional rotation prevention mechanism, the spring pin 107 is press-fit into and fixed to the upper cup 105, and a groove 102a is formed in the collar 102 so that the spring pin 107 engages with the groove 102a. The output shaft 101 is prevented from rotating by being connected to a Scotch yoke, etc. Further, the displacer 103 is prevented from rotating relative to the output shaft 101 by the spring pin 107. According to the conventional rotation prevention mechanism, the rotation of the displacer 103 inside the cylinder 100 is thus prevented.